Data from: Evolution and maintenance of haploid-diploid life cycles in natural populations: the case of the marine brown alga Ectocarpus

The evolutionary stability of haploid-diploid life cycles is still controversial. Mathematical models indicate that niche differences between ploidy phases may be a necessary condition for the evolution and maintenance of these life cycles. Nevertheless, experimental support for this prediction remains elusive. In the present work, we explored this hypothesis in natural populations of the brown alga Ectocarpus. Consistent with the life cycle described in culture, E. crouaniorum in NW France and E. siliculosus in SW Italy exhibited an alternation between haploid gametophytes and diploid sporophytes. Our field data invalidated, however, the long-standing view of an isomorphic alternation of generations. Gametophytes and sporophytes displayed marked differences in size and, conforming to theoretical predictions, occupied different spatio-temporal niches. Gametophytes were found almost exclusively on the alga Scytosiphon lomentaria during spring while sporophytes were present year-round on abiotic substrata. Paradoxically, E. siliculosus in NW France exhibited similar habitat usage despite the absence of alternation of ploidy phases. Diploid sporophytes grew both epilithically and epiphytically, and this mainly-asexual population gained the same ecological advantage postulated for haploid-diploid populations. Consequently, an ecological interpretation of the niche differences between haploid and diploid individuals does not seem to satisfactorily explain the evolution of the Ectocarpus life cycle.

单倍体-二倍体生活史的进化稳定性迄今仍存在广泛争议。数学模型研究显示，倍性阶段间的生态位分化可能是这类生活史演化与维持的必要条件，但该理论预测始终缺乏有效的实验佐证。本研究以褐藻水云属（Ectocarpus）的自然种群为对象，对上述假说展开验证。与实验室培养条件下已报道的生活史特征相符，法国西北部的克鲁安诺水云（Ectocarpus crouaniorum，下文简称E. crouaniorum）以及意大利西南部的硅质水云（Ectocarpus siliculosus，下文简称E. siliculosus）均呈现出单倍体配子体与二倍体孢子体的世代交替。然而，本次野外调查数据推翻了长期以来关于同形世代交替的固有认知。研究发现，配子体与孢子体体型存在显著差异，且契合理论预测，二者占据了截然不同的时空生态位：配子体仅在春季大量附着于绳藻（Scytosiphon lomentaria）之上，而孢子体则全年存在于非生物基质表面。悖谬的是，法国西北部的硅质水云种群虽无倍性阶段交替，却展现出相似的生境利用模式。该种群仅以二倍体孢子体存在，既可石生附生于岩石表面，亦可附生于其他生物体，这一主要以无性繁殖为主的种群，同样拥有理论中为单倍体-二倍体种群假定的生态优势。因此，仅通过单倍体与二倍体个体间生态位分化的生态学解释，似乎无法充分阐释水云属生活史的演化机制。